1. Field of the Invention
The present invention relates to a power storage device using an ionic liquid.
2. Description of the Related Art
Owing to an increase in demand for portable electronic devices such as a mobile phone and a laptop personal computer and development of an electric vehicle (EV) and the like, a demand for power storage devices such as an electric double layer capacitor, a lithium ion secondary battery, and a lithium ion capacitor has been significantly increasing in recent years. For the power storage devices, high capacity, high performance, safety in various operating environments, and the like are required.
To satisfy the above requirements, electrolyte solutions for the power storage devices have been actively developed. Cyclic carbonates are used for the electrolyte solutions for the power storage devices. In particular, ethylene carbonate is often used because of its high dielectric constant and high ionic conductivity.
However, not only an ethylene carbonate but also many other organic solvents have volatility and a low flash point. For this reason, in the case of using an organic solvent for an electrolyte solution for a power storage device, the temperature inside the power storage device might rise due to a short circuit, overcharge, or the like and the power storage device might burst or catch fire.
In consideration of the risks, the use of an ionic liquid, which is nonvolatile and flame-retardant, for an electrolyte solution of a power storage device has been studied. An ionic liquid is also referred to as a room temperature molten salt, which is a salt formed by the combination of a cation and an anion. Examples of the ionic liquid are an ionic liquid including a quaternary ammonium-based cation and an ionic liquid including an imidazolium-based cation (see Patent Document 1 and Non-Patent Document 1).